Tritium labeling process



TRITIUM LABELING PROCESS August Y. Mottlau, Railway, N..I., assignor to Essa Research and Engineering Company, a corporation of Delaware N Drawing. Filed Nov. 12, 1959, Ser. N 852,223 1 Claim. (Cl. 260-676) A technique of labeling hydrocarbons by placing themin intimate contact with curie amounts of tritium gas has been accomplished in the past by Riesz and Wilzbach, Journal of Physical Chemistry, 62, 6 (1958) and Wilzbach, I. Am. Chem. Soc. 79, 1013 (1957). In most cases, the tritiated product desired is the original parent compound changed only by having hydrogen atoms replaced by tritium atoms. However, a considerable amount of cracking or splitting of the hydrocarbon atoms occurs and several types of products of decomposition are produced. It is of course desirable that the tritiated parent constitute as high a proportion of the tritiated product as possible. In the petroleum industry where tracer compounds are extensively employed, it is generally desirable to employ liquid hydrocarbons in the labeling process and, for this reason, attempts have been made to minimize production of side products, particularly those which are normally gaseous, in labeling normally liquid hydrocarbons with tritium.

By the process of the present invention it has now been found possible to improve the degree of tritiation of a hydrocarbon while minimizing its breakdown. For example, normal hexane upon tritiation produces a tritiated C fraction of about 40% greater yield than has heretofore been attained. At the same time, the degree of tritiation of that C fraction has been increased as well. For example, the amount of tritiated C fraction expressed in millicun'es can be increased by about 77%.

In the present improvement, hydrocarbon either normally liquid or gaseous is tritiated in the presence of the noble gas, argon, employed in an amount up to a partial pressure approximately equal to the partial pressure of the tritium present for the purpose of improving the yield of the tritiated hydrocarbon product while minimizing the production of by-products. Using gaseous hexane, about 50% of the tritium introduced into hydro carbon is found in products more volatile than hexane. However, when using liquid hexane for the reaction, the utilization of tritium by fixation into desirable liquid hydrocarbon is nearly doubled, i.e., from 25% to 48%. It is probable that the active volatile molecular fragments formed in .the vapor phase by the prior technique react with tritium, which of course is plentifully present in that phase, with greater frequency than with other hydrocarbon fragments or molecules. On the other hand, although the present invention is not to be limited by a theory of its mechanism, in the liquid phase operation the opportunity for the tritiating of those volatile fragments is less.

The use of argon evens out the tritium radiation energy so that this energy is transferred to the normally liquid hydrocarbon at more moderate levels, and as a result there is a marked reduction in the formation of cracked or other extraneous products of the reaction. Ideally, thetn'tium shouldbe transferred 100% to the normally liquid hydrocarbon without the formation of any byproducts except hydrogen. The present invention, while not achieving the theoretically perfect transference of tritium, does improve upon the yields which were obtained by Riesz and Wilzbach, supra.

It is unnecessary to limit the hydrocarbon to be triti- 5 ated to one which is normally liquid. It is however desirable in practicing the instant novel process to carry out the reaction with at least a portion of the hydrocarbon being tn'tiated in the liquid phase. In other words, the use of super-atmospheric pressure and reduced temperatures, if necessary, maybe employed in order to maintain some liquid phase hydrocarbon for the tritiation re action. Hydrocarbon in both liquid .and vapor phase simultaneously, in liquid phase alone and in vapor or gas phase alone can be subjected to the novel tritiation reaction. 7 p 1 A series of experiments was undertaken employing normal hexane as the liquid hydrocarbon. The reaction was carried out at room temperature, and the closed containers were shaken for a period of two weeks while at room temperature.

Table 1 Sample No. I 1 n-Hexane 1 Tritium Argon (mL-liquid) (mm.Hg) (mm.Hg)

n-Hexane has a vapor pressure at room temperature of about 150 mm.Hg, which is the same as the partial pressure of the tritium. At the end of the reaction period, the hexane and the radiolysis products were frozen with liquid nitrogen and the fixed gases were pumped off. To minimize evaporation loss, samples were chilled with Dry Ice. Tritium distribution, as a C fraction, a lighter than C fraction, and a heavier than C fraction was determined for each of the three samples shown above using a radioassaying gas chromatograph. This instrument is similar to'that used by Riesz and Wilzbach, supra. The following re- 0 suits were obtained:

Table II Tritium Distribution Argon (Percent) Sample No. mru.Hg

O5 C5 C7 lllll'll These data indicate that dilution with argon increases the amount of tritium incorporated into the C fraction by almost 40% over the amount of tritium in the C fraction when employing no argon. By doubling the partial pressure of argon over that shown in sample 2, no further improvement in the tri 'ation selectivity was found. The tritium employed and as used in the containers had a radioactive energy level of about 3 curies. The efiect of the argon on the distribution of the tritiated products is apparent by studying the total millicuries of labeled hydrocarbons formed in each fraction. These values are obtained by multiplying a specific activity of each sample by the tritium distributions as shown in Table II. From the following table, it appears that a 1:1 argon-tritium dilution has little efliect on the C and lower compounds formed since only a slight reduction is indicated. The C and heavier compounds however are markedly increased. A further dilution with argon to 2: 1, as shown in sample 3, causes a pronounced and proportional decrease in the t-ritiation in each fraction.

Argon, up to a volume equalling that of the tritium, serves to render more efiicient and more uniform the distribution of the beta particles from the tritium. Part of the beta particle energy would be absorbed by the argon with the formation of ions (15.68 ev.) and meta-stable atoms (11.6 ev.) which, in turn, readily excite and ionize normal'hexane (10.43' ev.). The net result is that there is a larger number of hydrocarbon molecules excited, but there is less destructive violence involved and any overdose" of argon as for example in-sample 3, merely acts as an inert diluent assisting in the thermal dissipation of the energy. 7 i I i While the above s "'ples were all carried out using normal hexane, it is to be understood that any normally liquid hydrocarbon or mixture of hydrocarbons can be similarly employed and additionally, as stated heretofore, liquefied normally gaseous hydrocarbons may be similarly treated although their usefulness as tracer compounds in refinery operations is more hazardous to healthand also from the standpoint of ease'of handling is less advantageous than the use of the normally liquid tritiated hydrocarbons. The tracer techniques employed in refincry and chemical operations lend themselves more readily to the injection and tracing of normally liquid hydrocarbons, and these are preferred over the use of nonmally gaseous hydrocarbons.

The relative partial pressures as between the tritium and argon employed are generally about of equal degree. However, a lesser partial pressure may be employed and as sample number 3 illustrates, in some instances it is desirable to employ a greater partial pressure of argon than of the tritium, if the production of certain heavier fractions of hydrocarbons is desired.

Having now thus fully described and illustrated the character of the invention, what is desired to be secured by the Letters Patent is:

A process of tritiation which comprises contacting a normally liquid hydrocarbon a portion of which is in the liquid phase with tritium admixed with argon for a .suflicient length of time to effect tritiation of said hydrocarbon, the partial pressure of tritium employed at least substantially equaling the vapor pressure of said hydrocarbon measured at atmospheric temperature and the partial pressure of argon employed substantially equaling that of the tritium.

References Cited in the file of this patent Williams et al.: I. Am. Chem. Soc, 72 (1950), pages 5787-89 (only page 5788 relied on).

Fishman: J. Phys. Chem., 59 (1955), pages 469- 72 (only page 469 relied on).

Riesz et al.: J. Phys. Chem., 62 (1958), pages 6-9 (only page 6 relied on). 

